Rapid advancement in computer technology has enabled substantial miniaturization of portable personal computers and word-processors. In particular, so-called notebook computers no larger than standard office paper enjoy enormous popularity because of their compact size, light weight and low price.
The output display device for such computers is typically a liquid crystal display (LCD). Because liquid crystal displays are not light emitting, a light source is needed to display the LCD contents. While an external light source can be used, an internal light source is required when the external light source is too weak or where additional light is required for color display. Such an internal light source is called a backlight light source because light is radiated from the backside of the liquid crystal display, and must be a planar light source so that the entire liquid display screen can be illuminated. For this reason, a fluorescent discharge lamp combined with an electro-luminescent (EL) element or a lightguide plate is used as the backlight light source.
FIG. 1 shows an arrangement of a backlight light source using a fluorescent discharge lamp. In FIG. 1, reference numeral 1 represents a light guide plate made of a light transmitting material, such as glass or acrylic resin, provided with surface irregularities to radiate the light, which enters from the side of the plate in a planar direction. On both sides of light guide plate 1, fluorescent discharge lamps 2 are mounted as light sources to irradiate light guide plate 1.
In a common discharge lamp, gas capable of providing an electrical discharge is sealed in a tube formed of a transparent material such as glass. By applying an AC or DC voltage on discharge electrodes arranged face-to-face in the glass tube, an electrical discharge occurs via the gas and light is radiated to the outside.
In a fluorescent discharge lamp, low pressure (about 1 Pa) mercury (Hg) gas is used to provide the electrical discharge. Ultraviolet rays with a wave length of 253.7 nm are emitted from mercury gas to irradiate a fluorescent material, such as calcium halo-phosphate (3Ca.sub.3 (PO.sub.4).sub.2 CaFCl/Sb, Mn), which coats the inner wall of the glass tube. The ultraviolet rays are converted to visible light.
Also, argon (Ar) gas under pressure of several hundreds of Pa is introduced into the mercury gas to facilitate the electrical discharge by promoting ionization of the mercury gas (Penning effect).
FIG. 2a is a cross-sectional view of a fluorescent discharge lamp used for a conventional backlight light source. FIG. 2b is an enlarged cross-sectional view of the portion in the circle "b" of the tube end in FIG. 2a.
In these figures, reference numeral 21 represents a cylindrical, sealed glass tube container with an inner wall coated with a fluorescent material. On the left and right lateral ends of glass tube container 21, lead wires 22 are disposed. Tungsten (wolfram) filaments 23 coated with an electron emitting material such as barium oxide (BaO) are mounted on the tip of lead wires 22. Mercury dispensers 24 are arranged between filaments 23 and the tube ends.
In a conventional hot cathode, a tungsten filament coated with an electron emitting material such as SrO or CaO is usually adopted as an electrode material for a fluorescent lamp with the above arrangement. For a hot cathode, a preheating circuit is needed resulting in higher cost for the system. Also, a hot cathode consumes more power and the restriking voltage is high. Mercury ions generated during an electrical discharge are accelerated in the strong electric field in front of the cathode and collide with the electrodes. The collision splashes electrode material, leading to a phenomenon called sputtering. This shortens the service life of the electrodes and causes blackening of the tube ends near the electrodes.
To generate the mercury gas for electrical discharge, when manufacturing fluorescent discharge lamps, after the argon gas for starting the electrical charge is placed in the tube, the tube ends are closed to seal the entire tube. Then, mercury dispensers 24 are heated using a high frequency induction heater to decompose the Ti.sub.3 Hg sealed within it and discharge mercury vapor inside the tube. The mercury vapor thus discharged fills glass tube container 21 and generates ultraviolet rays via electrical discharge.
Because the filament serving as the hot cathode must have a certain size, it is not possible to reduce the inner diameter of the glass tube. The outer diameter of a normal glass tube is about 8 mm.
For notebook computers, in which a fluorescent discharge lamp is used as the backlight light source, there are strong demands for more compact design and energy-savings. This naturally leads to demands for an energy-saving and thin-type design of the backlight light source.
To cope with such demands, a cold cathode fluorescent discharge lamp without a filament has been proposed. FIG. 3 is an enlarged cross-sectional view of the tube end of such a lamp. In the cold cathode discharge lamp, cold cathode 25, also serving as a mercury dispenser, is mounted on lead wire 26 instead of the filament and mercury dispenser of the hot cathode discharge lamp shown in FIGS. 2a and 2b.
Since the cold cathode fluorescent discharge lamp has no hot cathode, unlike the hot cathode type fluorescent discharge lamp of FIG. 2, power consumption is low and the service life of the lamp is long. Because a filament is not used, it is possible to reduce inner diameter of the glass tube. The outer diameter of the glass tube is usually designed to be about 4 mm.
Nickel metal is used as the material for the cold cathode. Since nickel metal has a low electron emission property, it is not possible to increase the luminance, and the electrical discharge starting voltage is high.
U.S. Pat. No. 2,686,274 discloses a discharge electrode using ceramics, which is produced by turning ceramics such as BaTiO.sub.3 into a semiconductor by reduction processing. However, semiconductor ceramics in massive, granular or porous state are vulnerable to the impact of ions such as mercury ions and ions of rare gases including argon (Ar), neon (Ne), xenon (Xe), krypton (Kr), etc. Thus, the electron emission property deteriorates due to sputtering caused by the collision of ions.
To solve the above problems, a ceramic semiconductor electrode material having an anti-sputtering layer on the surface thereof and a method for manufacturing such an electrode material is disclosed in U.S. Pat. No. 4,808,883 Japanese Patent Laid-Open Publication 62-291854) and Japanese Patent Laid-Open Publications 55-49833, 2-186527, 2-186550 and 2-215039. However, there are still strong demands on material compositions having better properties as well as strong demands for more stable manufacturability.
A fluorescent discharge lamp electrode using the above ceramic semiconductor electrode material is disclosed in U.S. Pat. No. 4,808,883 (Japanese Patent Laid-pen Publications 62-291854, Japanese Utility Model Laid-Open Publications 63-15551, 63-15552, 63-15553, and 63-15554) and Japanese Patent Laid-Open Publications 2-186527-2-186550 and 2-215039.
The fluorescent discharge lamp electrode as described above comprises a solid ceramic semiconductor that has difficulty maintaining high temperature for electron emission. Japanese Patent Laid-Open Publication 4-43546 discloses a fluorescent discharge lamp electrode in which a ceramic semiconductor is formed in granular shape and is placed in a heat-resistant ceramic container to solve this problem.
FIG. 4a is a cross-sectional view of a ceramic electrode fluorescent discharge lamp disclosed in Japanese Patent Laid-Open Publication 4-43546. FIG. 4b is a cross-sectional view of a discharge lamp electrode. In FIGS. 4a and 4b, reference numeral 21 represents a glass tube containing argon gas and reference numeral 27 is an electrode cylinder. Glass tube 21 is a container having a cylindrical cross-section On the left and right lateral ends of glass tube 21, a lead wire 28, made of heat-resistant metal such as tungsten, is disposed and a retainer 29 for retaining electrode cylinder 27 is disposed on the tip of each lead wire 28. Retainer 29 is made of an elastic and electrically conductive material designed to elastically hold the outer periphery of electrode cylinder 27. Mercury dispenser 30 is provided in parallel to each lead wire 28, and a predetermined quantity of argon gas is sealed in the glass tube 21.
Electrode cylinder 27 comprises semiconductor ceramics with a closed bottom and one end open, and having a high melting point or good anti-sputtering property (e.g. semiconductor ceramics of Ba(Zr,Ta)O.sub.3 type). Cylinder 27 is elastically held by branches 31 disposed on retainer 29 at the end of lead wire 28. In a hollow portion 32 of the electrode cylinder 27, massive, granular or porous semiconductor ceramics 33 having an electron emission property are contained. On the surface of electrode cylinder 27, an anti-sputtering layer made of Ta is formed.
The size of electrode cylinder 27 is, for example, 0.9 mm in inner diameter, 1.9 mm in outer diameter, 23 mm in length, or 1.6 mm in inner diameter, 2.6 mm in outer diameter, and 2.3 mm in length.
Mercury dispenser 30 of the ceramic electrode fluorescent discharge lamp is placed outside retainer 29, adjacent in a radial direction to electrode cylinder 27.
In the ceramic electrode fluorescent discharge lamp, when the electrical discharge is started by the argon gas, ionized gas generates plasma near the discharge electrode, and the electron emitting semiconductor ceramics 33 are heated by the plasma. Thus, semiconductor ceramics 33 act as a hot cathode.
Because this ceramic electrode fluorescent discharge lamp has no filament, power consumption is low, and the problem of short life due to loss of electron emitting material via sputtering is avoided. Also, because it is of the hot cathode type, unlike the cold cathode type, it is possible to reduce the discharge starting voltage and to increase luminance.
However, mercury dispenser 30 is disposed at a position adjacent to electrode cylinder 27, and the outer diameter of the glass tube cannot be reduced. As a result, the requirements for the backlight light source of a notebook computer are not completely satisfied.